Mechanism for controlling propellers



May 15, 1951 D. BIERMANN MECHANISM FOR CONTROLLING PROPELLERS 3 Sheets-Sheet l Filed June 13, 1946 WM ...n

a NvENToR DAVID BIERMANN av May 15, 1951 D. BIERMANN MECHANISM FOR CONTROLLING PROPELLERS 3 Sheets-Sheet 2 Filed Junel 13, 1946 MGI m May 15, 1951 D. BIERMANN 2,553,128

MECHANISM FOR CONTROLLING PROPELLERS Filed June 13, 1946 3 Sheets-Sheet 5 FIG 'I loo 88 FIGIQ FIG. IO |06 |02. INVENTOR DAVID BIERMANN` BY |04 Ulummguwm ATTORNEYS Patented May 15, 1951 UNITED STATES- PATENT OFFICE MECHAISM'FOR CONTROLLING PROPELLERS David Biermann. Piqua,

Ohio, assigner to Hartzell Industries, Inc., Piqua,l Ohio, a corporation of .Ohio

Applicationlune 13, 1946 Serial No. 676,432

8 Claims.

This invention relates to aircraft, and particu-l larly to `method and mechanism for vcontrolling.

the pitch of adjustable pitch propellers.

The particular object .of this invention-lis to provide inccombination anadjustahle pitch pro peller, valve mechanism for controlling thesupn ply of actuatingfluid to the propellerpitch ad-` justing device and a control linkage therefor re-v motely operated from the plane cabin.-

Another object is to Yprovide in Ycombination with a propeller which is adjustable,y to `both positive and negative pitchfpositionsV ahydraulic actuating mechanism for adjusting fthe propeller,A

pitch and valve means controlled'ffrom the vcabin of the plane to control1the Supply oi uid to the` actuating mechanism.

Still another object is to provide independentactuating means for controlling thepositive and negative pitch positions, of an'adjustablev pitch propeller so that thereis` no possibility joi ,thev

pilot making an improperadjustmentf.

It is also an object ,toprovide ,anadjustabla pitch propeller having an. adjusting ',imechanism: controlled by linkage. extendingto .the pilotisv cabin, and having separate levers for controlling the positive and negative pitch positions, the lever controlling the negative pitchbeing normally locked in position.-

Still another object is to provide a diiierential adjustment of the propeller inpositive pitch, and* rapid adjustment of the propeller Vinto negative pitch.

These andother objects and advantages Will become more lapparent upon reference .to the following description ,and the accompanying `drawings in which:

Figure l is a View `of a plane illustrating theV location of the propeller -adjusting mechanism,

he actuating linkage. therefor and the operating levers in thepilots cabin;

Figure 2 is a fragmentary perspective View of an adjustable pitch propeller adapted for being controlled by the mechanism of this invention;

Figure 3 is a vertical section through the `propeller andiiydraulic vactuating mechanism therem for and is indicated suhstantiallyby the. plane,

(Cl. F10-460.37)

2.. Figure 4 is a plansection'taken on the line -t and illustrates the porting and.y lever varrangement in connection Withthef-hydraulic con# trol ,valve associated with the actuating mechanism for the propeller;

Figures 5', 6 and 7 are views-similar-toFigures but show-the control valve,vrespectively, in mini-*- mum positive,V maximum positive` and'negative' pitch positions;

Figure 8 is a view showing-,oneform ofilockV lever which may be employed in: connection with the negative pitch control means;

Figure9 is a fragmentary'view-showing a--rrrod.i V

ed arrangement of control valve Figure l0 isa section taken on the line ite-Iii` of Figure'9 and shows 'the' meansfoi'supplyingpressure uid tothe valvebore;

Figure l1 Vis a plan. section' 'indicated'by' the line I l--l l on Figure 9 and showstheinechanism in maximum positive pitch position?" Figure-12 is a View similar vte Figurefll showingthe mechanism in'maXi-mum. negative pitchrposition; and

Figure. 13 is a- Viewl showingthe mecltianism'Y moved Vtoward minimum. .pitchy -position from the positionshown in Figure l1.

General arrangement According to this-invention there is provided' an airplane propeller which is adjustable from-a positive-pitchposition into a negative pitch position. The'. propeller isY providedwith counter#v Weights which are responsive tov rotation of -the propellerby theV drive shaft to createa centrifugaliorce.;` This centrifugalforceV has a moment arm aboutthe axis of the propellerfblades by which it exerts a turningtorque on4 the said blades-tending to Vmove thematovvardf increased' pitch position.

For'movingthe propeller blades toward reduced positive pitch. position. orz intoY negative pitch,-

there is provided `a .hydraulic actuating .system which is operable to oppose the'.aforementioned= A- f servo-valvev moment ofcentrifugal Weights. is connected with the hydraulic actuatingmechanism so that the propellerv is positively retained in any position oi adjustment so-longas there is a supply of pressure fluid to the said valve.

For actuating th'e control member of the servovalve, there isa lever. systemwhich is controlled iromthe'cabinof the planeby the pilot.V This lever system consists of a rst-lever'm'eans hav-` ing a high ratio so that the said control member can oe lmoved in iine .adjustment'while ladjusting the propeller in posi-tive pitch;and-a .secondleverL Vmeans having a low ratio so that the propeller peller in positive pitch is located closely adjacent the pilot to be readily manipulated. The lever controlling the negative pitch position is positioned to be less available for actuation, as for example on the roof of the cabin, and is normally locked in position by a suitable means. Thus, while the pilot is able to adjust the propeller in positive pitch at any time, it is necessary to unlock one of the control levers before the propeller can be moved into negative pitch. This prevents the accidental shifting of the propeller in the wrong direction during the critical period of take-off or initial climbing of the craft.

Structural arrangement 'Referring to the drawings in detail, I designates a propeller blade having a shank or hub 2 recessed at 3 for receiving a drivingr shaft or sleeve 4, the other end ofwhich is mounted in the hub 5 in a recess 5. The hub 5 is mounted upon the engine shaft 'i by the spline portion S. The threaded end of the shaft 9 is provided with a lock nut I which interlocks with thehub 5 through the interlocking cone II. The hub is provided with a groove I2 having a lower shoulder I3 and an upper shoulder I4. Mounted in the hub adjacent to the lower shoulder I3 is a sealing ring I5, and above it a snap ring I5. The upper vshoulder I4 carries a split ring Il, which in turn engages with the upper ball race i8. The remainder of the ball bearing consists of the race I9 and balls 2B. This bearing is held in position by being engaged by the ange 2l of the clamping ring 22 which is made in' two parts and held together as shown in Figure 3 by the lower retain ing bolts 23 and the upper retaining bolts 24.

In assembly of the mechanism just described, the procedure is as follows. Seal I5 is slipped past its groove. The snap ring I6 is slipped into the groove normally occupied by seal i5. The ball bearing consisting of the parts I8, i 9 and 2E! is slipped over the sleeve 5. The split ring I'I is then placed in position. Snap ring I5 is then worked into position between shoulder I3 and race I9, after which the seal I5 is located in its groove. Thereafter, the shank 2 of the blade I is mounted upon the driving sleeve The split sleeve 22 is then placed around the assembly,

which causes the upper race I8 to fit snugly against the ring I1, which in turn ts against the shoulder I4. The bolts 23 and 24 are then at. tached and the halves of the clamp 22 are bolted together.

The hub 5 is provided at the left-hand end towards the engine with a cone sleeve 25 which engages with a cone locking ring 2E so that the hub is locked between this cone lock Ztl-2% and the cone locking ring Ii These details form no essential part of the invention but are recited in order to explain one type of mechanism for mounting the driving hub 5 on the engine shaft.

Counterbalancing mechanism One of the clamping halves of the clamp 22 is provided with a counterbalancing weight designated at 21 which extends outwardly and towards the axis of the drive shaft 1. This counterbalancing block is mounted by the screws 23 upon the inner end of one half of the clamp 22. It is 4 substantially radially disposedV with respect to the 'longitudinal axes of the Vpropeller blades. These counterbalancing weights move bodily with the blades as they rotate about the longitudinal axis of the shaft l, and under the Yinfluence of centrifugal force, tend to move the propeller blades in one direction. This direction is toward high pitch, and a hydraulic mechanism to be hereinafter described is utilized for decreasing the pitch.

It is generally preferable to have the hydraulic mechanism reduce the pitch so that if the uid pressure fails, the pitch will remain at the high limit. Thus, hydraulic pressure decreases pitch, while the centrifugal moment of the counterweight increases pitch.

First embodiment of hydraulic actuating mechanism The inner end of the clamp 22 is provided with a cap screw 29. Mounted on this cap screw and retained by the head on the cap screw is a thrust pin or pitman having an eye 3S mounted on the stud and having its base attached to the thrust ring 3|, which is attached by the key orscrew 32 to the inner ball race 33 of the ballbearing, including the balls 3Q and the outer race 35. The ring 3| is provided with a sleeve 35 which acts as a stop to halt the thrust ring in one position.

The ball bearing just described ts within the recess 3l in a piston 38 that has the sealing rings 39 and 39a and which reciprocates within a cylinder 49 bolted by the bolts lil on the stationari crankcase d2. The cylinder carries suitable guide pins, not shown, which guide the piston 38 in its reciprocation to prevent rotation thereof. It will be understood that while the ball bearing race 35 reciprocates bodily with the piston 38, the race 33, and the thrust ring 3l with its pitman thrust member 39 both reciprocate and rotate. Therefore, the'propeller is free to rotate with the shaft 'I of the engine but it may have the angularity of its blades adjusted according to the position of the hydraulic piston 38.

The control of the position of this piston in its movements is through the introduction and exhausting of hydraulic fluid into the cylinder i9 in the space 35, and the exhausting of fluid therefrom.

This is accomplished as follows. There is mounted on the face of the piston 38 a block 5l) having a horizontal passageway el registering with a horizontal passageway 48 in the piston 38, the'latter communicating with the space t5 behind the piston. The passageway Ii-'I is in communication with a vertical passageway 49 in the block which is intersected by a tube 5I which has a port 52 in registration with the passageway 49. It is apparent that the block 5G and the tube 5l move together with the piston 38. This tube is provided with diagonally disposed apertures 53 and 5d, which register, respectively, with the passageway 55 which communicates with a source of fluid pressure and with the passageway 55 which extends to an exhaust. The end of the tube 5i is suitably closed as by a plug 51. Surrounding the tube 5I is a reciprocating sleeve or servo-valve housing 5Ia which cooperates with the sleeve 5I to form a servo-valve. This valve member is connected by a linkage generally designated at 58 that extends into the cock pit of the plane to controls located adjacent to the pilots position. The valve member comprises the aforementioned ports 55 and 55 which are connected by suitable piping, preferably flexible tubing-f to s a s source. :or: hydraulicarpressure:

and to'r. an: exhaust, respectively.

face-tof:the..fleve1'=f6|1l andis .abutted by. thestop memberfl. so that. the 'controli means .fat 1D: is.;- stoppedin one directionyof movementqwiththe pivot; 68.in..its Figures 4,15 and 6 position` It will be-;.note'd .that thedistance between the. pivots'. S2 1 andgis.substantially less .than the .distance betweensthe `pivots $2 'and 54', :so that movement of thecontrolmeans at actuates the :valve meme berr' la :inwfine adjustmentrewhile movementl .of thei:ontroltmeans` at,.;'0"actuates the` valve membw la .quicldyvA Referring. howto-Figure .-1, itv willbe seen-that the .con-trol means ySleads throughxth'e.fuselage ofrthetplaneand into the cockpit or cabin thereof generally designated atM and is connected. With a-.shiftablecontrol lever 516.1.. This lever is posi.-

tioned- :adjacent the. pilot for easy manipulation. and; may comprise. .a plate .18 inscribed with; a l suitableiscalegso that thepilot can determine. the

exact4 setting. of the; propeller.

The control meansrfx'als'ojleads .throughthe fuselage of the plane to the cockpit or :cabin to belrzonnectedwithlan:actuatingrlevera.' However, the `leverf is positionedwhere normally' itwouldnotfbe grasped by thepilot; in vmistake-for any1-ot er-.controllejver. Itis `preferable. .that theirleverx be mountedsubstantially as shown above and somewhatto the rear ofthe usualzconf` trol-,levers and that there-be provided therewith stopped -tagainst'l thefstop member -12 whenthef--plane is tofland, and the pilotdesires to utilize the brakingteiect of` the .propeller in negative pitch; the lock screw ,Bernay ibe loosened andthe-lever Siirshifted to actuate thepi stonr38.v

vrso

andto moveV the propellerblades into their negative pitch position.-

InFigures 4,@5 and-6,l the lever 6U is-illustrated. in .several/positions :ofzpositive pitch and itk will benoted that the pivot 68 remains Iadjacent the stopfmember .12.j In Figure-'7, thev lever f6@- has -H been shiftedtoplace Athe propeller-'fin negative y pitch and it will be noted that thepvot 68- has:-

been: moved .asubstantial. distance awaytfrom the-stop member .12s.

Operation of the embodiment' of Figures 1 through 8 In operation, assuming that the plane is about-f position to :bring the stop gpin 68 against the stop brac-ketfl2 as shown-in Figures-5 and.A The leverfilis.. then locked. in position-bythe lock nut 86.. Thereafter, the pilotshifts `the lever 16"? into ,position -to givethe .propel-lerblades *.a some what reduced positive pitch This ris 'faccomv plished tas fol-lows z;

Shifting the lever 76 actuatesAthe-rod-S and,

through vits pivotal connection at 54j with .theleven 60;' shifts the saidleverffbytthe pvotvpin 68.. This avzplate. as,V at 32 havingaslot -84 byqmeans of. which, 'and ,the -lock-ing screw Bhf-.the vlever :can` befflocked in any predetermined iosition..A Nor--VV malin-the -lever' will beplocked withthe pivot' 68 Then,-

ing. :5 la. .throughjts pivotalronnectionzat 62 with.;`

thesaid levers.'A

Since-,theV movementoff the. propellerbladesfisff to be toward reduced pitch position the-.leven1.66,` will move from its-.Figure V6 position toward 'its Figurev 5 position.; This willmove the valve body;V 5 la 4ri'ghtwardlyand open the porti 53 to thelpreS-,fi

sure port 55. Pressure uidwil-l then flow'through the `hollow rod 5|, port-52S, and...conduit 49,-in'to the .cylindenspace 45. VThis will urge thepistonv 38 rightwardly and,xthr.ough ithe links intercom.- necting` the said pistonzwith the propeller blades, 1

rotate the blades toward reduced. pitch'position.

vsimultaneously, because. of` the. rigid connection between the: vsaid .piston`r and the; hollow .rod 5 I", the. rod followsthe valve body'v and again stops...

when thev port'53 is closed off from the :pressureporti-.55:y

Should it ybe desired to :place thefpropellerinu negativefptch, thepilot will release. the -.lever'\.801 by: unse rewing fthe lock 'nut 8 E. 1andshift the. lever.; so as to move the lever'f'into theuposition s shown. in '.Figure; 7.. At'. this' time. the valve ,body

5! will be shifted leftwardly and `again.introduce pressure .fluid throughl the port t3 to: the piston:

38 as mentioned above. Thisy willshiftthe; pro-` pellerf. blades past their.;centerarpositionfand 'into negative pitch.

Itzwillrbe. apparenti'thatatany time, .prior to the shifting of theV lever. te, ,thepilolthaefullconf troLover. theypositioning of :the propeller ,blades in..all of their. positivefpitch positionsr. Thus-,.-Y after the plane has taken off with the propeller;4

bladesinf vreduced;:pitch "position,. the .pilot/#cam by shifting the lever 3G.

Also7 dueto the fact that the connection of the lever Ell'with the valvebody, 5 la is much nearer the end of the said lever tov which the rod 'I0 is' attached than. to the; end ito whichfthe y.rad 65 is attached, the movementgofthe rod' 65 results;

in.-n.e adjustment :of theffyalve-body 5m, while movement :of the `rod r11 Ellwill .cause coarse,=-ade justinent or rapid movement; of; :the body 5 la.v Y,

Secondembodiment .of hydraulicactuating.

mechanism In Figures 9 through lthere is shownpa somewhat .modied arrangement-of. control valve .and4 actuating `.linkage bymeansaof Awhich' the pro`- peller `pitch-is controlledi Intheseviewssimilar. partsfbearisimilar.numbers with the addition off.

a subscript b..

valve lcasing 92.

crankcaseof the enginer as byftheebolts 93; and

the.A` port e6.- communicates` with the passage .l te" in; the-.crankcase Reference ,to `Fig-ure l0 will reveal the passage'. le! communicatesawith; af-second fpassage |02.;-

which is supplied withfpressure fluidirom any suitable sourceppreferablya ipump whichrdr'aws oil from the `cranlicase of; thegenginef. There'is; movement* of .the. lever. -69 shifts, the. valve heus-:- c y a SACI-leek Valve 7I U4 .pQStiOned betweenuthe parseTf sagesV |89 and |02 which is responsive to the fluid pressure in the passage |62 for permitting the flow of fluid into the passage l but prevents return flow.

Reciprocably mounted in the casing 92 is a valveV member |66 which includes the groove portion |38 which exactly spans the distance between the ports 94 and 96.

The valve member IBG is connected by the pin IIB with a lever ||2 which is slotted as at ||4 to receive the said pin. The lever I l2 has its upper end, as viewed in Figures 11, 12V and 18, pivoted at H6 to a link ||8.

.The lower end of the lever ||2 is pivoted at to a lever |22 which has its opposite end connected by the pivot |24 of a link |25. Between the-pivots |28 and |24, and substantially closer to the former, is a pivot point |28 toV which is Y tioned to abut the link I8 or the pivot IIB. The

link ||8 is for the purpose of adjusting the propeller in negative pitch while the link |26 adjusts the propeller in positive pitch. It will be noted from Figures l2 and 13 that the link |26 and lever |22 provide for ne adjustment of the propeller actuating piston while the link ||B and lever ||2 provide for rapid adjustment of the piston. l

It willbe noted that actuation of either of the links'i, ||8 move the valve member H36 relative to the valve casing 92, and that this movement actuates the piston 38h so that the lever system ||2, |22 is moved in a direction to return the valve member and valve casing to their original relative position, whereupon further movement of the piston stops.

Operation of the embodiment of Figures 9 through 13 The operation of the embodiment of Figures 9 through 13 is as follows: Y

The link |26 corresponds to the rod |56 in the first embodiment in that it adjusts the propeller blades in ne adjustment in positive pitch. The link ||8 corresponds to the rod l@ in that it effects adjustment of the propeller blades in negative pitch.

Movement of the link |28 rightwardly will cause pivoting movement of the rod |22 about the pivot |28 which is joined by the link i3@ with the block |34 carried on the face of the actuating piston 38h. The lower end of the link |22 thus moves leftwardly and causes the lever ||2 to pivot in a clockwise direction about its connection with the link I8. This moves the valve plunger |95 leftwardly and opens the port Si! to the port 96. Pressure iiuid is thus admitted to drive the piston 38h rightwardly and to reduce the pitch of the propeller blades. Rightward movement of the piston 38D will carry the block |34 rightwardly and, through the link |30, will move the lower end of the lever |22 rightwardly. This will carry the lever l2 in a counterclockwise direction about its pivotal support with the link ||3 and return the valve member H35 to its original position relative to the valve body.

'It will be apparent that movement of the link |26 is operable to cause lne adjustment in the pitch of the propeller blades between predetermined limits. If, now, the link ||8 is actuated,

it will pivot the lever ||2 about its pivotal connection at |20 with the lever |22. This will re-v sult in a rapid movement of the valve plunger IE6 and a correspondingly rapid movement of Yan approved control means for use in connection with an adjustable pitch propeller by providing for a iine adjustment in positive pitch and an adjustment operable for moving the propeller quickly into negative pitch. This invention provides for the further improvement of so locating the controlling means in the plane cabin 'that there is no likelihood of Athe pilot accidentally shifting the propeller into negative pitch when he desires Vto make a change in the positive pitch adjustment thereof. The mechanism employed for accomplishing the foregoing results is relatively simple, requiring a minimum of parts and being constructible at small expense.

It will be understood that this invention is susceptible to modification in order to adopt it to diierent usages and conditions and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

"I claim:

1. In combination; a propeller having blades rotatable on their axis; centrifugal means continuously urging said blades toward increasedY independent control levers for actuating the `mov- I able portion ofV said valve; and a diierential mechanism connecting said levers with the said movable portion whereby the rst thereof moves said member in fine adjustment and the other thereof moves said member rapidly. j

2. In combination; an adjustable pitch propeller having means continuously urging the baldes thereof toward maximum positive pitch position; iiuid operable means connected to actuate the blades toward negative pitch position; a follow-up servo valve controlling the supply of actuating fluid to said iiuid operable means; first and second separate and independent control levers for controlling said servo valve; and linkage means connecting said levers with said servo valve; said linkage means providing substantially different ratios of movement between said levers and said servo valve whereby one of said levers can adjust said valve in ne adjustment and the other thereof in coarse adjustment.

3. In combination; a variableV pitch propeller; means continuously urging said propeller toward maximum positive pitch position; a hydraulic plunger connectedto move the propeller blades through neutral and into' negative pitch position; a valve comprising a pair of relatively movable parts which control the supply of iiuid to and from said plunger; means connecting said valve with said plunger whereby movement of said piston is brought about by relative movement of said parts to return said valve parts to their initial position; a lever pivoted to one of said valve parts; and control rods for actuating the said one of said valve parts through said lever also connected with said lever at different distances from said valve member for obtaining different ratios of movement thereof.

4. In combination with a variable pitch propeller having means continuously urging the blades thereof toward maximum positive pitch; hydraulic means including a follow-up servo valve for moving said blades through neutral and into negative pitch position; an arm connected with one part of said valve and having a short extension to one side thereof and a long extension to the other side thereof; a rst control lever adjacent the pilots position connected to the end of said arm on the long side thereof; and a second control lever adjacent the pilots position but remote from the first lever connected to said arm on the short side thereof.

5. In combination with a variable pitch propeller having means continuously urging the blades thereof toward maximum positive pitch; hydraulic means including a follow-up servo valve for moving said blades through neutral and into negative pitch position; an arm connected with one part of said valve and having a short extension to one side thereof and a long extension to the other side thereof; a rst control lever adjacent the pilots position connected to the end of said arm on the long side thereof; a second control lever adjacent the pilots position but remote from the first lever connected to said arm on the short side thereof; and means of locking said second lever in position.

6. In combination with a variable pitch propeller having means continuously urging the blades thereof toward maximum positive pitch; hydraulic means including a follow-up servo valve for moving said blades through neutral and into negative pitch position; an arm connected with one part of said valve and having a short extension to one side thereof and a long extension to the other side thereof; a first control lever adjacent the pilots position connected to the end of said arm on the long side thereof; a second control lever adjacent the pilots position but remote from the rst lever connected to said arm on the short side thereof; stop means to abut and stop the said short side of said arm at a predetermined position; and locking means to lock said second control lever in position.

7. In combination with a variable pitch propeller having the blades thereof continuously urged toward maximum positive pitch; hydraulic means including a piston connected to move said blades through neutral and into negative pitch position; a valve having relatively movable parts hydraulically connected to control the supply of fluid to and from said piston; a first lever pivoted intermediate its ends to one of said valve parts; a second lever pivoted at one end to the end of said first lever; a rst control arm adjacent the pilots position connected with the free end of said second lever; a second control arm spaced from said rst control arm connected with the free end of said first lever; and means for returning said valve parts to their original relative position by said piston comprising a link connected between the said piston and one of said levers adjacent their connecting pivot.

8. In combination with a variable pitch propeller having the blades thereof continuously urged toward maximum positive pitch; hydraulic means including a piston connected to move said blades through neutral and into negative pitch position; a valve having relatively movable parts hydraulically connected to control the supply of iiuid to and from said piston; a rst lever pivoted intermediate its ends to one of said valve parts; a second lever pivoted at one end to the end of said rst lever; a first control arm adjacent the pilots position connected with the free end of said second lever; a second control arm spaced from said first control arm connected with the free end of said rst lever; and follow-up means for said valve comprising a drag link connected between said piston and said second lever adjacent to but spaced from its pivotal connection with said rst lever.

DAVID BIERMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 85,779 Atkins Jan. l2, 1869 1,154,591 .Dodge Sept. 21, 1915 1,630,810 Simpson May 31, 1927 1,816,787 Moss July 28, 1931 1,834,773 Fellmann Dec. 1, 1931 1,990,814 Castro Feb. 12, 1935 2,032,254 Caldwell Feb. 25, 1936 2,197,664 Mack Apr. 16, 1940 2,216,416 Mader Oct. 1, 1940 2,233,468 Barthel Mar. 4, 1941 2,234,003 Hruska Mar. 4, 1941 2,284,687 Schimanek June 21, 1942 2,425,261 Murphy et al Aug. 5, 1947 

